We evaluated the efect of in-vivo therapy with granulocyte colony stimulating factor (G-CSF) on neutrophils isolated from healthy adult volunteers being treated with this cytokine. Our hypotheses included that G-CSF therapy results in increased numbers of circulating neutrophils but has no enhancing efect on the microbicidal function of the circulting neutrophil and that the circulating neutrophil induced to mature under the effects of G-CSF therapy has unique functional, biochemical and survival characteristics which differ it from control neutrophils. We have not completed this study after examining aspects of neutrophil activity which included the NADPH oxidase system, granule content-as reflected by granule marker enzymes, survival in an ex-vivo assay, bactericidal activity, chemotaxis, F-actin content, expression of the surface B2-integrin CD11b, mRNA content and stimulus induced intracellular calcium.